A high voltage power source device used in a conventional image forming apparatus includes a digitally controlled piezoelectric transformer provided to each of transfer rollers for respective colors of black, yellow, magenta and cyan, and applies high direct current (DC) voltage to the transfer rollers for the respective colors (see Japanese Laid-Open Patent Application No. 2010-107608 (paragraphs [0020]-[0023] and FIGS. 1 and 12), for example).
However, in the conventional technology, if a plurality of piezoelectric transformers that generate high voltage by stepping up low voltage are provided on a single circuit board, secondary side high voltage outputs interfere with each other when drive pulse frequencies differ among the piezoelectric transformers. Therefore, a problem, such as occurrence of ripples in the high voltage output, occurs, causing a problem of unstable high voltage outputs. The present application considers a solution to the problem and has an object to produce a stable high voltage output when a plurality of piezoelectric transformers are provided in parallel with, and adjacent to, each other.